1. Field of the Invention
The present invention relates to a circuit board and a circuit module, and more specifically, relates to a circuit board and a circuit module each including an antenna coil.
2. Description of the Related Art
As an existing circuit board, for example, the flexible sheet described in International Publication No. 2007/015353 is known. Hereinafter, the flexible sheet will be described with reference to the drawing. FIG. 7 is a configuration diagram of the flexible sheet 500 described in International Publication No. 2007/015353.
As shown in FIG. 7, the flexible sheet 500 is one sheet formed from a flexible material, and is composed of coil pattern portions 500a to 500d and a mounting portion 501. Coil conductors 502a to 502d are provided on the pattern portions 500a to 500d, respectively. The coil conductors 502a to 502d constitute an antenna coil.
Further, a hard circuit board 600 is mounted on the mounting portion 501. Moreover, a semiconductor element 602 that performs a communication function for transmitting and receiving signals via the antenna coil to and from an external device is mounted on the circuit board 600. The flexible sheet 500 described above is mounted in an electronic device or the like in a state of being bent at dotted lines L1 to L4 (the dotted line L3 is indicated as a solid line since it overlaps the circuit board 600) in FIG. 7 and folded in a compact form. The flexible sheet 500 performs non-contact communication with an external device via the antenna coil to exchange signals.
Meanwhile, as described below, the flexible sheet 500 has a problem in that it is difficult to provide impedance matching between the semiconductor element 602 and the antenna coil, which is composed of the coil conductors 502a to 502d. Specifically, in the flexible sheet 500, the antenna coil is formed on the flexible sheet 500. The semiconductor element 602 is mounted on the circuit board 600 mounted on the flexible sheet 500. Thus, between the antenna coil and the circuit board 600, there are two connection portions, namely, a connection portion between the flexible sheet 500 and the circuit board 600 and a connection portion between the circuit board 600 and the semiconductor element 602. At the connection portions, a wiring width changes, and hence impedance matching is likely to deteriorate. Thus, in the flexible sheet 500, in order to provide impedance matching between the antenna coil and the semiconductor element 602, it is desired to reduce the number of the connection portions.